1. Field of the Invention
The present invention relates to a method for controlling the strength development of concrete, more particularly, predicting and controlling the strength development of concrete which undergoes special heat history, such as high and low temperatures. The present invention also relates to an apparatus for controlling the strength development of concrete.
2. Prior Art
Generally, relatively large concrete constructions called "mass concrete" tend to show higher temperatures of concrete after placement or deposition than ordinary concrete constructions. This tendency is more clear in the inside of the concrete constructions. In such mass concrete constructions, therefore, it is expected that strength development of the concrete at early ages becomes very high and that undesirable phenomena could occur which would adversely affect the strength of the concrete, such as cracking due to temperature differences between the environment and the mass concrete or within the inside of the mass concrete. Accordingly, the state of the strength development of concrete must be predicted and controlled appropriately by constructors.
When concrete constructions are being constructed under extraordinary or severe enforcement conditions, such as enforcement in hot and cold seasons, the strength development of concrete is greatly different from that of concrete constructions enforced under normal or mild conditions. Therefore, it is necessary to appropriately predict and control the state of strength development of concrete as stated above.
Conventional methods for determining the state of the strength development of mass concrete depending on the temperature history thereof include the following two major methods.
One of them is to collect at various times samples from actual size test concrete of the same composition as that of concrete to be used in an expected concrete construction. The test concrete was placed in water or air on the spot or placed in a standard water container kept at 20.degree. C., and the samples tested as by coring or core extraction experiments were performed to determine the strength of the test samples. This conventional method is disadvantageous especially when the concrete construction undergoes a special temperature history in that the great difference between the concrete construction and the test concrete lowers the accuracy of the method with the result that appropriate control of the strength development is difficult.
Another method is to determine the strength of concrete using a control system for controlling the strength of mass control as shown in FIG. 1. In FIG. 1, reference numeral 1a designates an actual mass concrete, i.e. an actually deposited concrete construction, and 1b is a heat evolution and heat conduction simulation system. The heat evolution and heat conduction simulation system 1b was made based on the supposition that if a one-dimensional rod model of concrete in the direction of the minimum size of the member is set up as shown in FIGS. 2a and 2b where C1 designates an imaginary concrete rod and boundary conditions at the time of construction, such as shuttering or exposition, are given as is on the both ends thereof followed by allowing concrete for heat evolution due to hydration of concrete and heat conduction or heat transfer, the model of concrete can automatically represent chronological temperature variation or temperature distribution in the direction of the minimum size of the member in the concrete. That is, if the above model can be materialized predictors of chronological temperature variation and temperature distribution in the direction of the minimum size of the member are directly available without performing experiments on various characteristics of concrete, such as the amount of heat evolution due to hydration, rate of heat evolution, heat conduction, heat transfer, etc.
A heat evolution and heat conduction system has already been proposed as described in Japanese Laid Open Patent Application No. 60-57252, which has a construction as shown in FIGS. 3 and 4. That is, concrete to be tested is deposited in an insulated tank 2 to form a concrete rod 3 having a cross-section of about 30 cm.times.30 cm. Proportional plus integral plus derivative action (P. I. D. Action) is performed so that the internal temperature of the concrete rod and the temperature conditions of the four surrounding surfaces of the insulated tank can coincide with each other. The temperature control is performed by a temperature control device 4 which measures the internal temperature of the concrete rod 3 using a plurality of C--C thermocouples 5 and sends instructions to a plurality of control heaters and heat transfer plates 6 in the four-sided insulated tank 2 so that they can always establish in real time the same temperature conditions as those detected.
With the above construction, there is formed in the four-sided insulated tank 2 a state in which heat conduction and heat transfer from the concrete rod in the direction of the surrounding 4 walls does not occur. Therefore, the concrete placed in the insulated tank 2 shows heat evolution due to hydration and heat conduction and heat transfer in the longitudinal direction (in the direction of the minimum size of the member).
As a result, automatic simulation of the heat evolution of mass concrete members due to hydration and heat conduction and heat transfer in the direction of the minimum size of the member is obtained. Predictors of chronological temperature variation and temperature distribution in the direction of the minimum size of the member can be obtained directly experimentally.
Reference numeral 1c designates a strength development control system for mass concrete, which comprises a control box 1d including a display device 1d.sub.1 for displaying the temperature or temperature difference, a temperature control device 1d.sub.2, etc., a water tank 1e adapted to place therein a test concrete member 1f, a thermocouple 1g for feeding back the temperature information and a heating unit 1h including a heater (not shown) for heating the test concrete member 1f and a fan (not shown), both the thermocouple 1g and the heating unit 1h being placed in the water tank 1e. In the thus-constructed control system 1c for mass concrete, the predictors of the temperature of the members output from the heat evolution and heat conduction simulation system 1b are obtained by the control device 1d.sub.2 in the control box 1d. The control device 1d.sub.2 automatically sends instruction to the heating unit 1g to enable it to give the same temperature conditions as the determined temperatures of the test concrete members 1f in real time, and as a result predictors of the state of the strength development of mass concrete members which are exposed to an unsteady high temperature state at early times can be obtained from the test concrete members in the water tank 1e.
The control system 1c can be operatively connected to C--C thermocouples embedded in an actually deposited mass concrete member at an appropriate interval to collect data on the temperature of the actually constructed mass concrete in order to use the data in the control of the strength development upon actual construction.
However, the above described conventional methods and apparatus are disadvantageous because the first method requires large-scale experiments which involve high costs. According to the second method, a large number of thermocouples must be arranged over a long distance sometimes as long as several hundred meters for large-scale constructions, such as a nuclear power plant and the like, and also because it is necessary to deposit concrete in the heat evolution and heat conduction system for giving the temperature process of mass concrete, which limits the conditions of experiments. Further, the conventional methods are also disadvantageous in that it was difficult to obtain accurate information on the temperature of concrete deposited in a long distance from the place of prediction and control especially when obstructions such as sea and rivers are present between the place of prediction and control of the strength development and the place of actual deposition of concrete.